[phobos] Failing unit tests on Windows

[David Simcha]

Don's gamma function code apparently fails some unit tests on Windows.  
I can't figure out why it passes on Brad's automated tester.  I'm using 
Windows 7 64-bit, Phobos revision 2181, and DMD 2.050.  I'm running the 
tests the standard way (make -fwin32.mak unittest).  I also tried 
running the unit tests just for the gamma function module in CodeBlocks, 
both with and without -O, -inline, and -release.  I get the following 
unit test failure:

core.exception.AssertError at std.internal.math.gammafunction(816): 
unittest failure

The relevant line of code is:

assert(betaIncompleteInv(0x1.ff1275ae5b939bcap-41, 4.6713e18, 
0.0813601)==0x1.f97749d90c7adba8p-63);